The invention relates to pulverulent, crosslinked polymers which absorb water, aqueous liquids and blood (superabsorbers) and have improved properties, in particular an improved retention and an improved retention capacity for liquids under pressure and an improved capacity for transportation of liquids, their preparation and their use as absorbents in hygiene articles and in industrial fields.
Superabsorbers are water-insoluble, crosslinked polymers which are capable of absorbing large amounts of aqueous liquids and body fluids, such as e.g. urine or blood, with swelling and the formation of hydrogels, and of retaining them under a certain pressure. As a result of these characteristic properties, these polymers are chiefly used for incorporation into sanitary articles, such as e.g. babies"" nappies, incontinence products or sanitary towels.
The superabsorbers which are currently commercially available are substantially crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, in which some of the carboxyl groups are neutralized with sodium hydroxide solution or potassium hydroxide solution.
For aesthetic reasons and from environmental aspects, there is an increasing trend to make sanitary articles, such as babies"" nappies, incontinence products and sanitary towels, ever smaller and thinner. To ensure a constant total retention capacity of the sanitary articles, this requirement can be met only by reducing the content of large-volume fluff. As a result of this, further tasks fall to the superabsorbers in respect of transportation and distribution of liquid, which can be summarized as permeability properties.
In the case of superabsorber materials, permeability is understood as meaning the capacity for transportation and three-dimensional distribution of added liquids in the swollen state. This process proceeds in the swollen superabsorber gel by a capillary transportation through intermediate spaces between the gel particles. Transportation of liquid through swollen superabsorber particles itself follows the laws of diffusion and is a very slow process which plays no role in the distribution of the liquid in the use situation of the sanitary article. In superabsorber materials which cannot effect capillary transportation because of a lack of gel stability, separation of the particles from one another, avoiding the gel blocking phenomenon, has been ensured by embedding these materials into a fiber matrix. In nappy constructions of the new generation, there is only little fiber material, or none at all, in the absorber layer to assist in transportation of the liquid. The superabsorbers used here must accordingly have a sufficiently high stability in the swollen state so that the swollen gel still has a sufficient amount of capillary spaces through which liquid can be transported.
To obtain superabsorber materials with a high gel strength, on the one hand the degree of crosslinking of the polymer can be increased, which necessarily results in a reduction in the swellability and the retention capacity. An optimized combination of various crosslinking agents and comonomers, as described in the patent specification DE 196 46 484, is indeed capable of improving the permeability properties, but not to a level which allows, for example, incorporation of a layer which optionally comprises only superabsorbers in a nappy construction.
Methods for surface after-crosslinking of the polymer particles can furthermore be used. In so-called after-crosslinking, the carboxyl groups of the polymer molecules on the surface of the superabsorber particles are reacted with various after-crosslinking agents which can react with at least two of the carboxyl groups close to the surface. In addition to increasing the gel strength, the ability to absorb liquid under pressure is greatly improved in particular, since the known phenomenon of gel blocking, in which swollen polymer particles stick together and as a result a further absorption of liquid is prevented, is suppressed.
The surface treatment of liquid-absorbing resins is already known. To improve dispersibility, ionic complexing of the carboxyl groups close to the surface with polyvalent metal cations is proposed in U.S. Pat. No. 4,043,952. The treatment is carried out with salts of polyvalent metals dispersed in organic, optionally water-containing solvents (alcohols and other organic solvents).
An after-treatment of superabsorber polymers with reactive surface-crosslinking compounds (alkylene carbonates) to increase the absorptive ability for liquids under pressure is described in DE-A40 20 780. A surface after-crosslinking of superabsorbent polymers with polyfunctional crosslinking agents, such as polyvalent metal compounds, in the presence of an inert, inorganic powder, such as SiO2, to improve the absorption properties and to produce a non-tacky gel of the polymer particles is described in DE-A-35 03 458.
According to EP-A-0 574 260, superabsorbent polymers with a low residual monomer content which does not change decisively even by surface crosslinking are obtained if certain conditions are maintained during the polymerization and the after-crosslinking is carried out with conventional polyfunctional crosslinking agents, such as polyols, alkylene carbonates and polyvalent metal salts, under conventional conditions.
The after-crosslinked polymers show a good absorption without the use of pressure. According to EP-A-0 889 063, superabsorbent polymers which are already surface-crosslinked can be treated against free-radical degradation by body fluids, in particular L-ascorbic acid, by after-treatment with a compound of titanium or zirconium and a compound which chelates these metal compounds.
EP 0 233 067 describes water-absorbing resins which are crosslinked on the surface and are obtained by reaction of a superabsorbent polymer powder with 1-40 wt. %, based on the polymer powder, of an aluminium compound. A mixture of water and diols, which is said to make the use of lower alcohols as solvents superfluous, is used as the treatment solution. 100 parts by wt. of crosslinking agent solution are applied to 100 to 300 parts by weight of absorber. The diols (e.g. polyethylene glycol 400 and 2000, 1 ,3-butanediol or 1,5-pentanediol) added to the reaction medium of water also serve to prevent lumping together of the superabsorber during treatment with the large amounts of aqueous treatment solution used here. The solvent is removed in a subsequent drying step at 100xc2x0 C. The polymers treated in this way have an inadequate level of properties, and an improvement in the absorptive ability under pressure is not achieved. Furthermore, treatment with large amounts of treatment solution cannot be carried out economically in modem, continuously operating processes.
WO 96/05234 describes a process for the treatment of superabsorbent polymers, according to which the surface of the absorber particles, which contain at least 10 wt. % water, was treated with a crosslinked layer obtained by a reaction of a reactive, hydrophilic polymer or a reactive organometallic compound with an at least bifunctional crosslinking agent at temperatures below 100xc2x0 C. Metal salts are not mentioned. The metal compounds employed must be able to react with the functional groups of the crosslinking agent. Organometallic compounds are therefore recommended as the metal compounds, and should be present in a weight ratio of 0.1 to 30 to the crosslinking compound. The polymers obtained are said to have a balanced ratio of absorption, gel strength and permeability, the measurement values stated being determined under less critical conditions. Thus, for example, the absorption and the permeability are determined without any pressure loading. A disadvantage of this known process is the use of solvents and toxically unacceptable crosslinking reagents, such as e.g. the polyimines, alkoxylated silane or titanium compounds and epoxides mentioned as preferred.
According to WO 95/22356 and WO 97/12575, an improvement in the permeability and liquid transportation properties is achieved by an appropriate treatment of commercially obtainable superabsorber polymers with amino polymers in organic solvents. The serious disadvantage of the process described here, in addition to the use of toxicologically unacceptable polyamines and polyimines, lies in the use of large amounts of organic solvents, which are necessary for the treatment of the polymers. The associated safety aspect and cost outlay rules out a production on a large industrial scale. In addition to the toxicological unacceptability of these treatment agents, it is furthermore to be taken into account that they also tend to decompose under the high after-crosslinking temperatures, which manifests itself, inter alia, in a yellowing of the absorber particles.
To prepare water-absorbing polymers with an improved abrasion resistance, the Japanese laid-open specification JP-A-09124879 is directed to after-crosslinking of the surface with polyfunctional crosslinking agents, the water content of the polymer particles again being adjusted to 3-9 wt. % after the surface-crosslinking and it being possible for this amount of water to contain inorganic compounds, such as metal salts.
Superabsorbent polymers which, according to WO 98/48857, are brought into contact in particle form with polyvalent metal salts by dry mixing and are then provided with a certain amount of a liquid binder, such as water or polyols, are said to have an improved gel blocking during absorption of aqueous liquids. The polymer particles can be subjected to after-crosslinking of the surface before this treatment.
To minimize the tendency of superabsorbent after-crosslinked polymer particles towards agglomeration due to electrostatic charging, WO 98/49221 recommends re-moistening of the polymer particles to the extent of up to 10 wt. % water with an aqueous additive solution. These aqueous solutions can contain mono- or polyvalent ions or propoxylated polyols. It is also possible for the polymer particles already to be brought into contact with the aqueous additive solution before the after-treatment of the surface, as a result of which a more uniform distribution of the agent for after-treatment of the surface is said to be achieved.
No indication that, while retaining a high retention capacity and absorptive ability for liquid under pressure in the after-crosslinking stage, the permeability properties can also be increased drastically is to be found from the prior art described above.
The object of the present invention is to provide superabsorbent polymers which have an improved combination of properties, in particular not only a high absorptive ability under pressure, but also combine the conventionally opposing properties of a high retention capacity and a good permeability, i.e. have a level of the combination of properties at which, in addition to a retention value of at least about 25 g/g, at least an SFC value of at least, 45xc2x710xe2x88x927, preferably at least 50xc2x710xe2x88x927 cm3 sec/g is present. In particular, an object is to provide superabsorbent polymers which are suitable above all for use in very thin nappy constructions with a very high superabsorber content. For this case, polymers with retention values of at least about 25 g/g and permeability values of SFC more than 70xc3x9710xe2x88x927 cm3 s/g are required in particular.
A further object of the invention was to discover preparation processes for such superabsorbent polymers which can be carried out simply, economically and reliably, give a uniform product quality and in which in particular small amounts of solvent are used and where possible organic solvents are avoided. It should furthermore be possible to carry out the processes without the use of toxicologically unacceptable substances.
The object according to the invention is achieved by providing a pulverulent polymer which is after-crosslinked on the surface, absorbs water, aqueous or serous liquids and blood and is built up from
a) 55-99.9 wt. % of polymerized, ethylenically unsaturated monomers which contain acid groups and are neutralized to the extent of at least 25 mol %,
b) 0-40 wt. % of polymerized, ethylenically unsaturated monomers which can be copolymerized with a),
c) 0.1-5.0 wt. % of one or more polymerized-in crosslinking agents,
d) 0-30 wt. % of a water-soluble polymer the sum of the amounts by weight of a) to d) being 100 wt. %, the polymer has been coated with
e) 0.01 to 5 wt. %, based on the polymer, of at least one polyol as an agent for after-crosslinking of the surface in an aqueous solution and with
f) 0.001-1.0 wt. %, based on the polymer, of a cation in the form of a salt dissolved in an aqueous solution and has been heated to an after-crosslinking temperature in the range from about 150 to 250xc2x0 C., the weight ratio of salt to polyol being in the range from 1:0.8 to 1:4 and the total amount of aqueous solutions having a range from 0.5 to 10 wt. %, based on the polymer, excluding crosslinked partly neutralized polyacrylic acid which has been treated with Al2(SO4)3 and glycerol in a weight ratio of 1:1 or with Al2(SO4)3 xc2x716H2O and polyethylene glycol in a weight ratio of 1:1.8 or with Al2(SO4)3xc2x714H2O and ethylene glycol in a weight ratio of 1:2 or with Al2(SO4)3xc2x718H2O and ethylene glycol in a weight ratio of 1:2 or Al2(SO4)3xc2x718H2O and propylene glycol in a weight ratio of 1:1.6.